We propose to carry out studies on the operation and control of the purine nucleotide cycle in skeletal muscle, brain, kidney. The cycle will be studied in cell free extracts, in perfused organs (skeletal muscle and kidney), and in situ (skeletal muscle and brain). The effects of specific inhibitors of adenylate deaminase, adenylosuccinate synthetase, and adenylosuccinase will be investigated. The object of these experiments is to obtain insight into the functions which have been proposed for the purine nucleotide cycle. The regulatory properties of the highly purified enzymes of the purine nucleotide cycle will be investigated with special emphasis on physiological concentration ranges of substrates, activators, and inhibitors. The control and function of 5'-nucleotidase will be studied with some emphasis on the enzyme from heart. Ths enzyme catalyzes the production of adenosine, which we believe to be an extracellular signal of the intracellular energy status. The hypothesis will be tested that 5'-nucleotidase catalyzes a vectorial reaction in which intracellular AMP is expelled as extracellular adenosine.